This patent application claims priority based on Japanese patent application H10-18987 filed on Jan. 30, 1998, the contents of which are incorporated by reference.
1. Field of Invention
The present invention relates to a soliton pulse compression optical fiber and a method for making a soliton pulse compression optical fiber.
2. Description of Related Art
Optical soliton transmission is a technology which uses an optical soliton to transmit a large number of signals over a long distance. A soliton is a solitary wave that acts like a particle. When an optical soliton is propagated within an optical fiber, the optical soliton maintains its original waveform over a long distance. To transmit a large number of signals by optical soliton transmission, the width of the optical soliton pulse has to be small.
The dispersion decreasing fiber, DDF, is reported in the proceedings of the 1995 communications society conference of IEICE, B-731, to be capable of decreasing the width of an optical soliton pulse which is propagated within an optical fiber. The dispersion decreasing fiber has the characteristic of decreasing dispersion in the longitudinal direction of the optical fiber, that is, along the length of the optical fiber.
The method for making the dispersion decreasing fiber disclosed in Japanese Patent Application Laid-Open No. H7-157324 is to cut the clad of the optical fiber preform so as to make a rod which is taper-shaped in the longitudinal direction, and to pull the optical fiber preform in such a manner as to produce an optical fiber having a uniform diameter.
FIG. 1 shows the relationship among the cutoff wavelength xcexc, the dispersion D, and the modal field diameter MFD of the dispersion decreasing fiber made by cutting the clad of the optical fiber preform. The larger the cutoff wavelength xcexc is, the smaller the clad thickness is. And the smaller the cutoff wavelength xcexc is, the larger the clad thickness is. The cutoff wavelength xcexc is the wavelength which determines whether the optical fiber performs as a multi-mode type fiber or as a single-mode type fiber. If the wavelength of injected light is longer than the cutoff wavelength xcexc, the optical fiber performs as a single-mode fiber. And, if the wavelength of injected light is shorter than the cutoff wavelength xcexc, the optical fiber performs as a multi-mode fiber.
Dispersion, D, is a phenomenon whereby the width of a light pulse increases as the light pulse is propagated through an optical fiber. The dispersion is a measure of the broadening of the group delay time per 1 km of fiber length, and per 1 nm of spectral width of light source. The unit of dispersion is expressed as ps/nm/km. If the maximum value of the intensity distribution in an optical fiber is 1, the diameter corresponding to an intensity distribution of 1/e2 is the modal field diameter, where e is the base of the natural logarithm, e=2.71.
It is desirable to decrease the dispersion occurring along the length of a dispersion decreasing fiber from 13.7 ps/nm/km to 2.3 ps/nm/km so as to compress light pulses having large widths. The regions where the dispersion decreases from 13.7 ps/nm/km to 2.3 ps/nm/km correspond to regions 20 and 22 in FIG. 1. When the dispersion is 13.7 ps/nm/km, the modal field diameter becomes as large as 6.6 xcexcm, a region 20. When the modal field diameter becomes large, the bending loss also becomes large. Bending loss occurs when light leaks from the core to the clad when an optical fiber is bent. Light cannot propagate through an optical fiber when the bending loss is large.
On the other hand, if light having a wavelength of 1.55 xcexcm is used and the cutoff wavelength is set to be less than 1.55 xcexcm, the cutoff wavelength becomes larger than 1.55 xcexcm when the dispersion becomes larger than 2 ps/nm/km. Thus, the dispersion decreasing fiber cannot be used as single-mode fiber at any band where the cutoff wavelength is less than 1.55 xcexcm.
Therefore, it is an object of the present invention to provide a soliton pulse compression optical fiber and a method for making a soliton pulse compression optical fiber which is capable of solving the problems described above.
According to a first aspect of the present invention, the soliton pulse compression optical fiber for compressing the injected light pulse in width comprises a core and a clad that surrounds the core wherein a maximum relative index difference between the core and the clad is between about 1.2% and 2.5%. According to a second aspect of the present invention, the soliton pulse compression optical fiber has a maximum relative index difference between the core and the clad between about 1.2% and 1.5%. According to a third aspect of the present invention, the soliton pulse compression optical fiber has a maximum relative index difference between the core and the clad of about 1.5%.
According to a fourth aspect of the present invention, the soliton pulse compression optical fiber has a core diameter that varies along the length of the soliton pulse compression optical fiber. According to a fifth aspect of the present invention, the core of the soliton pulse compression optical fiber has a step-like refractive index distribution.
According to a sixth aspect of the present invention, the optical fiber preform of the soliton pulse compression optical fiber comprises a core preform, which is to be a core of soliton pulse compression optical fiber, and a clad preform that surrounds the core preform, wherein the maximum relative index difference between the core preform and the clad preform is between about 1.2% and 2.5%.
According to a seventh aspect of the present invention, the method for making an optical fiber preform of a soliton pulse compression optical fiber comprises the steps of preparing a core preform to be the core of the soliton pulse compression optical fiber and forming a clad preform that surrounds the core preform, wherein the maximum relative index difference between the core preform and the clad preform is between about 1.2% and 2.5%
According to an eighth aspect of the present invention, the method for making a soliton pulse compression optical fiber comprises the steps of preparing a core preform to be the core of the soliton pulse compression optical fiber, forming a clad preform that surrounds the core preform with a maximum relative index difference between core preform and clad preform of between about 1.2% and 2.5%, and pulling the soliton pulse compression optical fiber preform, wherein the diameter of the soliton pulse compression optical fiber varies in the longitudinal direction.
According to a ninth aspect of the present invention, the method for making a soliton pulse compression optical fiber results in a soliton pulse compression optical fiber having a maximum relative index difference between the core and the clad of between about 1.2% and 1.5.%. According to a tenth aspect of the present invention, the method for making a soliton pulse compression optical fiber results in a soliton pulse compression optical fiber having a maximum relative index difference between the core and the clad of about 1.5%.
According to an eleventh aspect of the present invention, the method for making a soliton pulse compression optical fiber further has a core diameter which varies along the length of the soliton pulse compression optical fiber. According to a twelfth aspect of the present invention, the method for making a soliton pulse compression optical fiber has a step-like refractive index distribution.